Someone downloaded my game but it failed on Game Start due to code that doesnt exist

As stated in the title, they get the following error:

 ############################################################################################  FATAL ERROR in  action number 1  of Other Event: Game Start  for object obj_music_controller:   ds_map_find_value argument 1 incorrect type (undefined) expecting a Number (YYGI32)  at gml_Script_scr_checkLastLoggedInJson  ############################################################################################  --------------------------------------------------------------------------------------------  stack frame is  gml_Script_scr_checkLastLoggedInJson (line -1)  gml_Object_obj_music_controller_Other_2  

The function ds_map_find_value() is not being called in the script scr_checkLastLoggedInJson(). (Even if it was, how could it be happening at line -1?)

Nor is it called in the Game Start event of the obj_music_controller object. I also checked every use of ds_map_find_value() in the entire game and they are all fine.

Furthermore, this is only happening with 1 person. And they have tried uninstalling it and re-downloading. Even after i uploaded a new version of the game, this still happened to them and only them.